[improve] refactor nest block to improve readability by guard and extract

[Thespica]

What's changed?

As #2056 describes. But it's still working in progress.

Checklist

• [x] I have read the Contributing Guide
• [ ] I have written the necessary doc or comment.
• [ ] I have added the necessary unit tests and all cases have passed.

Add or update API

• [ ] I have added the necessary e2e tests and all cases have passed.